Monday, February 26, 2024

Firing Primitive Pottery

Pre-heating around a premier fire
Cow-dung fire
Fired wild-clay objects with carbon staining
Wood fire -- attempt to burn off the carbon
Sadly, even more Carbon staining Some science behind all of this. Ignore the kiln comments, they do not apply to my firing method · Jeff Zamek · · “The pot fills the gap between art and life.” Philip Rawson, Chubut, Argentina What’s happening when heating and cooling my kiln? If you make ceramic sculpture or pottery and look at a firing kiln this question must come to mind. As the temperature increase reactions are occurring and clay and glazes are changing in irreversible ways. The pottery that comes out of your kiln will be the last objects left when our sun ends its life and consumes the earth. 212 0 F. – 392 0 F. 100 0 – 200 0 C. Mechanical water is removed from the clay particle surfaces. 842 0 F. – 1112 0 F. 450 0– 600 0C. Chemical water is driven off as in the clay formula Al203 2Si02 2H20. During the mechanical and chemical water removal range if the kiln is fired too fast pots can crack or blow up often damaging adjoining pottery. 1067 0 F. – 575 0 C. Quartz in the clay goes through a sudden expansion at the same time that shrinkage occurs in the 842 0 F. to 1112 0F. - 450 0C. to 600 0C. If the kiln is fired up too fast or cooled too fast during this range pots can crack. 572 0 F. 1292 0 F. – 300 0 C. 700 0 C. Sintering, Earthenware Firing the kiln in complete oxidation will remove organic matter in the clay. Excess oxygen and time will promote a clean bisque firing. Incomplete oxidation can leave carbon in the clay resulting in bloating at higher temperatures. 1922 0 F. 2012 0 F. - 1050 0 C. -1100 0 C. Spinel (a group of minerals of the same structure and formula) develops from the metakolin changing to mullite needles. Feldspar starts to melt into a glassy phase dissolving silica from kaolin forming mullite. The clay body becomes dense and vitreous. 2192 0 F. – 1200 0 C. Vitrification? Feldspar continues melting into a glass phase decreasing porosity in the clay body. 2012 0 F. 2282 0 F. - 1100 0 C. 1250 0 C. Stone ware?. Any quartz in the clay body changes to cristobalite while other fluxes in the clay body and feldspar aids in the quartz to cristobalite conversion. On the cooling cycle cristobalite inversion occurs around 392 0 F. -200 0 C. If the kiln is opened prematurely at this range some glaze pinging can be heard. Additionally, wood fired kilns and kilns with low B.T.U. capacity frequently slow down or stall out after cone 8 (2280 0 F. – 1249 0 C.) and can develop excessive amounts of cristobalite which can cause pots cracking in typical oven ware temperatures. Kiln Insulation – Commercially made kilns are designed with the appropriate thermal gradient in terms of insulation properties. When planning any kiln building project investigate the insulation options for the type of kiln being built. Under Insulated Kilns that are under insulated can be expensive to fire due to extensive heat loss during the firing. Under insulated kilns might not reach temperature or take excessively long to achieve clay body and glaze maturing temperatures. In enclosed kiln rooms there is the potential problem of the potter’s safety due to high kiln room temperatures. There are clay body and glaze defects caused by firing kilns too fast or too slow (see other blogs). Over Insulated Conversely, kilns constructed with high levels of insulation can coast. If you have noticed at the end of a firing that pyrometric cone 9 is at the 3 o’clock position bending and the next day when opening the kiln the cone is at the 4 o’clock position the cone has been subject to “heat work” which is the result of temperature over time. When the heat source has been turned off the cones are in the maturing range longer due to the greater degree of kiln insulation. In some instances this situation can cause glazes to run or appear over fired in color and/or texture. Matte glazes can appear glossy or satin matte while glazes containing high percentages of metallic coloring oxides can run on vertical surfaces or pool in horizontal surfaces. Slower cooling rates can promote crystal growth in glazes. Porcelain clay bodies can stick to the kiln shelve or deform when subjected to prolonged times in their maturing temperature zones. This is due to their highly vitreous composition as opposed to stoneware and earthenware clays containing lower glass formation. Dense “accordion pleated” packed fiber kilns and kilns constructed of only hard bricks can be over insulated subjecting pots to excessive heat work in the cooling cycle. Hard brick kilns take a long time to heat and cool due to the density of the brick which has to be first heated to then radiate heat to the pots. Hard brick kilns take longer times to cool due to their increased thermal bank radiating heat. When to unload the kiln Under ideal conditions it is a good policy to let the kiln cool to room temperature before opening and unloading the pots. Not many of us do that! As a safe alternative a bisque kiln can be unloaded below 300 0 F. – 148 0 C. as the pots are not dense enough to be thermally shocked. However, be aware the kiln shelves, posts and pots can still be hot enough to burn you. Potters and bakers frequently have burn scars on their forearms from reaching into the kiln and ovens burning themselves on the still hot shelves. When opening a glaze kiln try to unload below 200 0 F. – 93 0 C. but if you still hear glaze pinging close the kiln and wait till the temperature drops below 100 0F. – 37 0 C. These unloading guidelines can be adjusted depending on your level of risk however, it is always best to wait when in doubt. Information sourced from, Ceramic Science for the Potter by W.G. Lawrence, Professor Lawrence taught at Alfred University

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